The pharmacogenomic profiles of drug metabolizing enzymes play an important role in pharmacokinetics (PK) of drugs. Phenobarbital (PB), worldwidely used for neonatal seizure, is a drug that requires careful dose adjustments based on therapeutic drug monitoring. It was reported that phenobarbital (PB) metabolism was affected by CYP2C9 and CYP2C19 polymorphisms in adults. This study aims to evaluate the effects of the CYP2C9 and CYP2C19 genetic polymorphisms on PB PK in infants with neonatal seizure for an optimal dosing strategy.